Many significant advances in the art of microwave drying have been described and claimed in several prior patents assigned to the assignee of the present invention. Among these, for example, are U.S. Pat. Nos. 3,707,773, titled "Multi-Line Gluing of Superimposed Leaves," and 3,756,889, titled "Glue Fastening of Superimposed Leaves." These prior patents discuss some of the more difficult problems encountered in the production of fine-quality, multi-leaved business forms and, to a certain extent, the solutions set forth in said patents to the problems are relevant to the invention herein described and claimed. However, as will be apparent as the present description proceeds, the principles of the present invention are not limited to use in connection with the production of business forms or to the drying of glue lines, whether used in connection with business forms or not. Rather, the principles herein have wide-ranging utility and may be applied in such situations, for example, where it is necessary to dry ink on continuously moving webs so that the webs can be folded or otherwise handled immediately after printing without running the risk of smearing the ink. Hence, it should be kept in mind throughout the description which follows that the illustration in the drawings of a multi-leaved business form having glue lines which must be dried, and the use of such business forms in the description which follows, are by way of example only.
It is known in the art to utilize a serpentine or "meandering" wave guide in connection with microwave drying. Such installations provide the wave guide with a plurlity of serially interconnected guide sections that extend across the path of travel of the moving web and are adapted to expose the web to the microwave energy within the various sections as the web progressively advances from one end of the guide to the other. Hence, a leading part of the web is exposed not only to the microwave energy within the first encountered guide section, but also to that within the subsequently encountered guide sections, thereby prolonging the overall exposure of the web to microwave energy and hence promoting rapid drying. It has been found, however, that it is not unusual for the standing waves produced within the guide sections to orient themselves in such a way that the several wave peaks in each section are aligned with corresponding wave peaks in the other sections, thereby forming lines of wave peaks extending longitudinally of the web and spaced apart according to the frequency being utilized. Such wave peaks represent points of maximum energy, and hence it might be thought of as the peaks presenting "hot spots" which can be applied to the moving web for drying purposes. When such hot spots are lined up in the aforesaid manner, the areas between the spots remain virtually unaffected, to the end that uneven drying across the width of the web can result.
This is particularly significant where the web being dried consists of a multi-leaved business form or a series of interconnected, snap-out envelopes which require crossweb glue lines as opposed to glue lines only along the longitudinal edges of the moving web. It can be appreciated that if microwave energy is applied only to certain spots along the crossweb glue lines, the glue line will be unevenly dried, leading to the serious problem of "tenting" that arises adjacent the cross-perforation lines in the web where folding is to occur. Such tenting consists in the separation of leaves at the line of fold, forming a small triangular bulge that simply cannot be tolerated because of the resulting malfunction of the high-speed folding equipment with which such webs are utilized.